FIG. 1 is a schematic circuit diagram illustrating a capacitive touch panel system according to the prior art. As shown in FIG. 1, the capacitive touch panel system comprises plural driving units u1˜u6, plural sensing circuits s1˜s6 and a touch panel. The touch panel comprises plural driving electrodes d1˜d6 and plural receiving electrodes r1˜r6, which are not directly connected with each other. The driving electrodes d1˜d6 are connected to output terminals of respective driving units u1˜u6. The receiving electrodes r1˜r6 are connected to input terminals of respective sensing circuits s1˜s6. In addition, mutual capacitances Cs11˜Cs66 are existed between the driving electrodes d1˜d6 and respective receiving electrodes r1˜r6. For clarification, six driving electrodes d1˜d6 and six receiving electrodes r1˜r6 or the touch panel are shown in FIG. 1. The capacitive touch panel with more driving electrodes and more receiving electrodes may have the similar configurations, and will not be redundantly described herein.
The capacitive touch panel of FIG. 1 is a multi-finger touch panel. When a conductive pointed object (e.g. a finger) touches the capacitive touch panel, the mutual capacitance value is changed. According to the change of the mutual capacitance value, a touched position is realized. Generally, once the finger of user is placed on a touch point of the capacitive touch panel, the mutual capacitance value at the touch point is changed. Meanwhile, a driving signal is sent to the corresponding mutual capacitance. In response to the driving signal, the electric quantity stored in the mutual capacitance is correspondingly changed. Based on this characteristic, the change of the electric quantity is detected by the sensing circuit. That is, according to the change of the voltage signal, the change of the mutual capacitance value is realized. According to the change of the mutual capacitance value, the sensing circuit may judge whether a pointed object approaches or touches the capacitive touch panel. Moreover, since the relationship between electric quantity (Q), voltage (V) and capacitance value (C) complies with the formula Q=C×V, the sensing circuit may also provide a voltage change to a backend circuit. The backend circuit may realize the position of the touch point according to the voltage change.
Please refer to FIG. 1 again. The six driving signals P1˜P6 will sequentially provide respective pulses to the driving electrodes d1˜d6 through the driving units u1˜u6. Since the mutual capacitances Cs11˜Cs66 are existed between the driving electrodes d1˜d6 and respective receiving electrodes r1˜r6, the coupling charge of the mutual capacitances Cs11˜Cs66 will be transmitted to the sensing circuits s1˜s6 through the receiving electrodes r1˜r6. As such, output voltages Vo1˜Vo6 are respectively outputted from the sensing circuits s1˜s6.
For example, the pulse of the first driving signal P1 generated in a driving cycle T will charge the mutual capacitances Cs11˜Cs16, which are existed to the first driving electrode d1. In addition, the coupling charge of the mutual capacitances Cs11˜Cs16 will be transmitted to the sensing circuits s1˜s6 through the receiving electrodes r1˜r6. Correspondingly, output voltages Vo1˜Vo6 are respectively outputted from the sensing circuits s1˜s6.
Assuming that the touch point is near the mutual capacitance Cs11, the output voltage Vo1 outputted from the first sensing circuit s1 is different from the output voltages Vo2˜Vo6, which are respectively outputted from the sensing circuits s2˜s6. Whereas, assuming that two touch points are respectively near the mutual capacitances Cs11 and Cs16, the output voltages Vo1 and Vo6 outputted from the first sensing circuit s1 and the sixth sensing circuit s6 are different from the output voltages Vo2˜Vo5, which are respectively outputted from the sensing circuits s2˜s5.
In the next driving cycles, the driving signals P2˜P6 sequentially provide pulses to the driving electrodes d2˜d6. Correspondingly, output voltages Vo1˜Vo6 are respectively generated by the sensing circuits s1˜s6.
In this example, these six driving cycles T are considered to constitute a scanning cycle τ. In other words, after the scanning cycle τ, all areas of the capacitive touch panel have been scanned once. As such, the position of the at least one touch point on the touch panel can be realized.
FIG. 2 is a schematic circuit diagram illustrating a sensing circuit of the capacitive touch panel system according to the prior art. As shown in FIG. 2, the sensing circuit s is implemented by an integrator. The sensing circuit s comprises an operation amplifier 200 and a feedback capacitor Cf. A reference voltage Vref is inputted into the positive input terminal (+) of the operation amplifier 200. Both terminals of the feedback capacitor Cf are respectively connected to the negative input terminal (−) and the output terminal Vo of the operation amplifier 200. In addition, the negative input terminal (−) of the operation amplifier 200 is also connected to the receiving electrode r. A mutual capacitance Cs is connected between the receiving electrode r and a driving electrode d.
During normal operation of the operation amplifier 200, the voltages inputted into the positive input terminal (+) and the negative input terminal (−) of the operation amplifier 200 are both equal to the reference voltage Vref. In a case that the amplitude of the pulse passing through the driving electrode d is Vy, a voltage change ΔVo at the output terminal Vo is obtained.
The voltage change ΔVo is calculated by the formula (I): ΔVo=−Vy×Cs/Ci. Take the first driving signal P1 shown in FIG. 1 for example. In a case that no touch point is created, the mutual capacitance values of the mutual capacitances Cs11˜Cs16 are unchanged, and thus the voltage changes at the output terminals Vo1˜Vo6 of the sensing circuits s1˜s6 are identical. On the other hand, when the touch point is near the mutual capacitance Cs11, the mutual capacitance value of the mutual capacitance Cs11 is changed, and thus the voltage change at the output terminal Vo1 of the first sensing circuit s1 is different from the voltage changes at the output terminals Vo2˜Vo6 of the sensing circuit s2˜s6. According to the voltage changes at the output terminals Vo1˜Vo6 of the sensing circuit s1˜s6, the backend circuit may realize the position of the touch point.
However, if the change of the mutual capacitance value of the mutual capacitance Cs at the touch point is very small, the coupling charge of the mutual capacitance Cs is slightly different from the coupling charge of other mutual capacitances. As such, the voltage change generated by the sensing circuit corresponding to the touch point is slightly different from the voltage changes generated by other sensing circuits. Under this circumstance, the backend circuit fails to realize the position of the touch point according to the change of the voltage change.
FIG. 3 is a schematic circuit diagram illustrating another capacitive touch panel system according to the prior art. As shown in FIG. 3, since each of the driving signals P1˜P6 has two sub-driving cycles (t1, t2) during each driving cycle T, the coupling charge of the mutual capacitances of the capacitive touch panel 300 can be generated in several times. In other words, the sensing circuits s1˜s6 may be designed to accumulate the coupling charge of the mutual capacitances in several times. As such, the output voltages Vo1˜Vo6 from the sensing circuits s1˜s6 will be easily distinguishable.
As shown in FIG. 3, a scanning cycle τ includes six driving cycles T, and each of the driving signals P1˜P6 generates two pulses during the sub-driving cycles t1 and t2, respectively. In such way, the coupling charge of the mutual capacitances will be generated in several times. The sensing circuits s1˜s6 are designed to accumulate the coupling charge of the mutual capacitances in several times and generate a higher voltage change for determining the position of the touch point. In other words, after the scanning cycle τ, all areas of the capacitive touch panel have been scanned once. Consequently, the position of the at least one touch point on the touch panel can be accurately realized.
For clarification, as shown in FIG. 3, two sub-driving cycles (t1, t2) are included in each driving cycle T. It is noted that more than two sub-driving cycles may be included and more than two pulses may be generated during each driving cycle T. As such, the sensing circuits s1˜s6 generate a higher voltage change. The use of multiple pulses to accumulate the coupling charge of the mutual capacitances is disclosed in for example U.S. Pat. No. 6,452,514, which is entitled “Capacitive sensor and array”.